Field of the Invention
The present invention relates to image forming apparatuses, such as a copier, a laser beam printer and a facsimile, and particularly to image forming apparatuses that form an image using a plurality of laser beams.
Description of the Related Art
To realize high speed image forming and high image quality, image forming apparatuses that include a multi-laser light source provided with a plurality of light emitting elements emitting laser beams have been increasing in the market. The image forming apparatus using the multi-laser light source can form an image at high speed, and output an image with high quality.
Some multi-laser light sources include a plurality of light emitting elements arranged in series at prescribed intervals. The light emitting elements are disposed such that different positions on a photosensitive member in the rotational direction of the photosensitive member are exposed to laser beams emitted from the respective light emitting elements. In a recent image forming apparatus, to form an image with a high resolution, the multi-laser light source is rotationally adjusted when the apparatus is assembled. This adjustment allows exposure position intervals (exposure spot intervals) of laser light in the rotational direction of the photosensitive member to be in conformity with the resolution of the image forming apparatus.
Japanese Patent Application Laid-Open No. 2001-13434 discloses that positional deviations of exposure spots created by laser beams in the main scanning direction in which the laser beams scan a photosensitive member are corrected by adjusting light emission timing of each laser. The light emission timing is adjusted depending on an optical property of scanning light from a laser scanner unit (optical scanning unit; hereinafter, called “LSU”) that includes a multi-laser light source provided with a plurality of light emitting elements. The deviations of pixels due to positional deviations of exposure spots of respective laser beams in the main scanning direction are corrected. This correction can suppress the relative deviation of pixels formed by the respective laser beams in the sub-scanning direction, which is the rotational direction of the photosensitive member. Expensive dedicated components, such as a sensor capable of responding at high speed and a focusing lens, are required to realize the invention of Japanese Patent Application Laid-Open No. 2001-13434. Accordingly, adoption of such a configuration in every image forming apparatus causes a problem of cost. Thus, in a conventional art, when an LSU is manufactured, the optical property of scanning light is measured using a measurement jig simulating an ideal position of the photosensitive drum, and an adjustment value for light emission timing of each laser, which is derived from the measurement result, is recorded in a ROM. In an actual use, the image forming apparatus reads the adjustment value from the ROM, adjusts the light emission timing, and forms an image.
Each of the LSU and the photosensitive drum is installed at prescribed positions in the image forming apparatus. In the case of adjusting the positional deviations of exposure spots based on the laser emission timings of the respective lasers as with Japanese Patent Application Laid-Open No. 2001-13434, the LSU embedded in the image forming apparatus cannot be adjusted correctly unless the relative positions of the embedded LSU and the photosensitive drum are ideal. In the case where the light emission timings cannot be adjusted correctly, an exposure spot row is inclined from the sub-scanning direction at a prescribed angle; this inclination is called inclination deviation. In a recent multi-laser that has a number of lasers, the distance between the opposite ends of plurally arranged lasers is large. Accordingly, adverse effects of the inclination deviation are large. For correct adjustment of laser light emission timing, an electric adjustment method is inexpensive. However, in this method, writing adjustment factors and magnification adjustment factors coexist as much as the number of lasers. Accordingly, adjustment cannot easily be performed.
In view of these conventional problems, it is a main object of the present invention to provide an image forming apparatus that can adjust an inclination deviation even after an LSU and a photosensitive member are embedded.